1. Field of the Invention
The present invention relates to a CMOS image sensor, and more particularly to a CMOS image sensor that permits an increase in light sensitivity through amplification of pixel detection signals, without an accompanying enlargement of circuit devices.
2. Description of the Related Art
CMOS image sensors have attracted attention as low cost image sensors. CMOS image sensors can be manufactured by means of an ordinary CMOS process, are characterized by low power consumption, low voltages and a low cost, and have been adopted in inexpensive digital cameras and the like in place of CCD sensors.
FIG. 1 shows the overall configuration of a typical CMOS image sensor. This image sensor comprises a pixel array 10 in which pixels having a photoelectric conversion circuit are arranged in the form of a matrix; a row select circuit 12 for selecting pixels oriented in the row direction; a sample-and-hold circuit SH for holding pixel signals outputted from selected pixels to column lines; and a column select circuit 14 for selecting pixel signals held by the sample-and-hold circuit SH and outputting these signals to an output line 16. Further, analog pixel signals outputted to the output line 16 are amplified by an amplifying circuit 20 and then converted to digital pixel signals (to pixel data) by an A/D conversion circuit 22.
FIG. 2 is a part detail circuit diagram of a conventional CMOS image sensor. An in-pixel photoelectric conversion circuit, and sample-and-hold circuits for each column are illustrated in this figure. Also, pixels P00 to P11, which form two rows and two columns, are shown, the in-pixel photoelectric conversion circuit being shown only for pixel P00.
The in-pixel photoelectric conversion circuit is provided with a photodiode PD, which is a photoelectric conversion element, a reset transistor N1, an amplifier transistor N2, and a select transistor N3. The row select circuit 12 drives the reset line RST such that the reset transistor N1 conducts, whereby the node npd connected to the photodiode PD is pre-charged to a reset voltage VRST level. When the reset transistor N1 is non-conductive, a current corresponding to the amount of light received is generated by the photodiode PD and the potential of the node npd drops in accordance with this current. The voltage drop of the node npd corresponds to the pixel signal. Further, when, after a fixed duration during which light is received, the row select circuit 12 drives a row line ROW0 such that the select transistor N3 is caused to conduct, the pixel signal, which is amplified by the transistor N2, is outputted to a column line CL0. A current source transistor N4 is connected to the column line CL0.
Sample-and-hold circuits SH0, SH1 are connected to column lines CL0, CL1 respectively and temporarily hold pixel signals which have been detected by pixels and then amplified. The sample-and-hold circuits SH0, SH1 illustrated constitute examples of CDS (Correlated Double Sampling) circuits. The CDS circuits comprise a capacitor Csh for holding analog image signals outputted to column lines CL1, CL2, analog amplifying circuits 30, 34 being provided before and after the respective capacitor Csh respectively. Operation of these CDS circuits will be described hereinafter.
In a conventional CMOS image sensor, the node npd signal, which corresponds to the image signal in the pixel, is amplified by the source follower transistor N2 and then held by a sample-and-hold circuit SH. A source follower circuit can only output a signal amplitude on the order of 0.8 times the signal amplitude of the detection node npd to the source terminal, and does not have an adequate amplification capacity. Consequently, as shown in FIG. 1, an amplifying circuit 20 is provided outside the pixel array and amplifies detected pixel signals. However, because signals which have been photoelectrically converted by the pixels are not amplified directly, such signals comprise noise and the like, which precludes outputting of image signals with sufficient sensitivity.
Meanwhile, consideration has been given to the provision of an amplifying circuit as an enhancement to an in-pixel circuit. However, when such an amplifying circuit is provided within a pixel, the number of devices in the pixel increases such that the numerical aperture of the image sensor (=the area of the photodiode÷pixel area) falls, which is disadvantageous.